One use of sensor arrays is to isolate signal components that are traveling from, or propagating to, a particular direction. They find use in a number of different applications. For example, sonar systems make use of sensor arrays to process underwater acoustic signals to determine the location of a noise source; arrays are also used in radar systems to produce precisely shaped radar beams. Array processing techniques for isolating received signals are known as beamforming and when the same or analogous principles are applied to focus the transmission of signals, the techniques are referred to as beamsteering.
Considering the process of beamforming in particular, it is typically necessary to use a fairly large number of signal processing components to form the desired directional beams. The signal from each sensor is typically divided into representative components by subjecting each signal to multiple phase shift, or time delay, operations which cancel the equivalent time delay associated with the respective relative position of the sensor in the array. To form the directional beam the time shifted signals from each sensor are then added together. The imparted time delays are chosen such that the signals arriving from a desired angular direction add coherently, whereas those signals arriving from other directions do not add coherently, and so they tend to cancel. To control the resulting beamwidth and sidelobe suppression, it is typical for each time delayed signal to be multiplied or xe2x80x9camplitude shadedxe2x80x9d by a weighting factor which depends upon the relative position of the sensor in the array.
Beamforming in one dimension can thus be realized through a relatively straight-forward implementation using a linear array of sensors and a beamforming processor, or beamformer, that delays each sensor output by the appropriate amount, weights each sensor output by multiplying by the desired weighting factor, and then sums the outputs of the multiplying operation. One way to implement such a beamformer is to use a tapped delay line connected to each array element so that the desired delay for any direction can be easily obtained by selecting the proper output tap. The beam steering operation then simply consists of specifying the appropriate tap connections and weights to be applied.
However, a beamforming processor becomes much more complex when a two dimensional sensor array is used. Not only does the number of time delay operations increase as the square of the size of the array, but also the physical structures required to connect each sensor to its corresponding delay becomes complex. At the same time, each delay unit must be provided with multiple taps for the formation of multiple beams. The problem can become prohibitively complicated when the simultaneous formation of multiple beams is required.
As to implementation choices, beamforming technology was originally developed for detection of acoustic signals in sonar applications. The beamformers built for these early sonars used analog delay lines and analog signal processing components to implement the sum and delay elements. Networks of resistors were then used to weight and sum the appropriately delayed signals. However, the number of beams that can be implemented easily with such techniques is limited since each beam requires many discrete delay lines, or delay lines with many taps and many different weighting networks. As a result, it became common to share a delay line by using scanning switches to sequentially look in all directions. However, with this approach only one beam is available at a given time.
Recent advancements in integrated circuit electronics has provided the capability to implement practical digital beamforming systems. In these systems a signal from each sensor is first subjected to analog to digital conversion prior to beamforming. The beamformers are implemented using digital shift registers to implement the delay and digital multiplier components to implement the required weighting. The shift registers and multiplier components are typically controlled by command signals that are generated in general purpose computers using algorithms or equations that compute the values of the delays and phase weightings necessary to achieve the desired array beam position. Beam control thus requires fairly complex data processors and/or signal processors to compute and supply proper commands; this is especially the case if more than one beam is to be formed simultaneously.
For these reasons, few multi-dimensional multiple beam systems exist that can operate in real time with a minimum implementation complexity.
The invention is a beamsteering or beamforming device (generically, a beamforming device), that carries out multi-dimensional beamforming operations as consecutive one-dimensional operations. In a preferred embodiment the two operations are interposed by a transpose operation. For example, beamforming for a two-dimensional array of sensors is carried out as a set of projections of each desired output beam onto each of the two respective axes of the array.
Signal samples are periodically taken from each sensor in the array and then operated on as a group, or matrix, of samples. A first one-dimensional (1D) beamformer is used to form multiple beams for each sensor output from a given row of the sample matrix. The multiple output beams from the first 1D beamformer are then applied to a transposing operation which reformats the sample matrix such that samples originating from a given column of the sensor array are applied as a group to second one-dimensional (1D) beamformer.
The beamformer can be implemented in an architecture which either operates on the samples of the sensor outputs in a series of row and column operations, or by operating on the sample matrix in parallel. In the serial implementation, a group of multiplexers are used at the input of the first 1D beamformer. Each multiplexer sequentially samples the outputs of the sensors located in a given column of the array. The multiplexers operate in time synchronization such that at any given time, the outputs from the group of multiplexers provide samples from the sensors located in each row of the array.
The multiplexers then feed the first 1D beamformer that calculates the projection of each row onto a first array axis, for each of the desired angles. In the serial implementation, the first 1D beamformer is implemented as a set of tapped delay lines formed from a series of charge coupled devices (CCDs). Each delay line receives a respective one of the multiplexer outputs. A number of fixed weight multipliers are connected to predetermined tap locations in each delay line, with the tap locations determined by the set of desired angles with respect to the first array axis, and the weights depending upon the desired beam width and sidelobe suppression. Each output of the first 1D beamformer is provided by adding one of the multiplier outputs from each of the delay lines.
The serial implementation of the transposer uses a set of tapped delay lines with one delay line for each output of the first 1D beamformer. The tapped delay lines have a progressively larger number of delay stages. To provide the required transpose operation, samples are fed into the delay lines in the same order in which they are received from the first 1D beamformer; however, the samples are read out of the delay lines in a different order. Specifically, at a given time, the output of the beamformer are all taken from a specific set of the last stages of one of the delay lines.
Finally, the second 1D beamformer consists of a set of tapped delay lines, fixed weight multipliers and adders in the same manner as the first 1D beamformer. However, the weights and delays applied by the second 1D beamformer are determined by the set of desired angles to be formed with respect to a second axis of the array.
In a parallel implementation of the invention, the multiplexers are not used, and instead the outputs of the array are fed directly to a set of parallel processing elements which operate on samples taken from all of the sensors simultaneously. Each processing element produces a set of beamformed outputs that correspond to the samples taken from one of the rows of sensors beamformed at each of the desired angles with respect to the first array axis. In this parallel implementation, the transposing operation is carried out by simply routing the outputs of the processing elements in the first 1D beamformer to the appropriate inputs of the second 1D beamformer. The second 1D beamformer likewise is implemented as a set of parallel processing elements, with each processing element operating on beamformed samples corresponding to those taken from one of the columns of the array, beamformed at each of the desired angles with respect to the second array axis.
In another preferred embodiment of the invention, a low power time domain delay and sum beamforming processor involves programmable delay circuits in sequence to provide a conformal acoustic lens. This electronically adjustable acoustic conformed lens has a plurality of subarrays that can be separately controlled to adjust viewing angle and their outputs coherently summed for imaging.
The invention provides a substantial advantage over prior art beamformers. For example, a device capable of steering up to one hundred beams for a ten by ten sonar array can be implemented on a single integrated circuit chip operating at a relatively low clock rate of 3.5 MegaHertz (MHZ), representing a continuous equivalent throughput rate of approximately 14 billion multiply-accumulate operations per second.